Impact of Elevated Carbon Dioxide on Primary, Secondary Metabolites and Antioxidant Responses of Eleais guineensis Jacq. (Oil Palm) Seedlings

A split plot 3 by 3 experiment was designed to investigate the relationships among production of primary metabolites (soluble sugar and starch), secondary metabolites (total flavonoids, TF; total phenolics, TP), phenylalanine lyase (PAL) activity (EC 4.3.1.5), protein and antioxidant activity (FRAP) of three progenies of oil palm seedlings, namely Deli AVROS, Deli Yangambi and Deli URT, under three levels of CO(2) enrichment (400, 800 and 1,200 µmol·mol(−1)) for 15 weeks of exposure. During the study, the treatment effects were solely contributed by CO(2) enrichment levels; no progenies and interaction effects were observed. As CO(2) levels increased from 400 to 1,200 µmol·mol(−1), the production of carbohydrate increased steadily, especially for starch more than soluble sugar. The production of total flavonoids and phenolics contents, were the highest under 1,200 and lowest at 400 µmol·mol(−1). It was found that PAL activity was peaked under 1,200 µmol·mol(−1) followed by 800 µmol·mol(−1) and 400 µmol·mol(−1). However, soluble protein was highest under 400 µmol·mol(−1) and lowest under 1,200 µmol·mol(−1). The sucrose/starch ratio, i.e., the indication of sucrose phosphate synthase actvity (EC 2.4.1.14) was found to be lowest as CO(2) concentration increased from 400 > 800 > 1,200 µmol·mol(−1). The antioxidant activity, as determined by the ferric reducing/antioxidant potential (FRAP) activity, increased with increasing CO(2) levels, and was significantly lower than vitamin C and α-tocopherol but higher than butylated hydroxytoluene (BHT). Correlation analysis revealed that nitrogen has a significant negative correlation with carbohydrate, secondary metabolites and FRAP activity indicating up-regulation of production of carbohydrate, secondary metabolites and antioxidant activity of oil palm seedling under elevated CO(2) was due to reduction in nitrogen content in oil palm seedling expose to high CO(2) levels.